A so-called GRIN lens is a cylindrical graded index lens (GRaded INdex lens) having a refraction index distribution within glass so as to bend a traveling direction of light, thereby realizing the function of the lens. Typically, the GRIN lens is cut into pieces of less than about 1 mm, each of which is fused to the tip of an optical fiber, and used in a variety of applications in optical systems for optical communications. Today, there is a demand for GRIN lenses having high NA (numerical aperture). Preforms of such GRIN lenses are fabricated in a sol-gel method as disclosed in Patent Document 1 below and the like. A preform that is fabricated using the sol-gel method has a cylindrical shape whose diameter and length are respectively about 5 mm and 30 to 50 mm at the minimum. This size is very small compared to that of a preform for an ordinary optical fiber.
When the preform is drawn into the GRIN fiber lens, a fiber diameter is controlled as follows. That is, a diameter measuring instrument that measures the fiber diameter of the drawn GRIN lens fiber is provided outside a heating furnace that heats the preform so as to elongate the preform. A measured value (fiber diameter) is transmitted to an automatic controller and used to control a drawing speed such that the fiber diameter becomes closer to a target value.
Patent Document 2 below discloses a method for controlling an outer diameter of a preform when a silica glass ingot for an optical fiber is elongated so as to form a preform. According to this technology, diameter measuring instruments are provided at two positions in the middle of a part of the preform being elongated in a heating furnace. An elongation speed is controlled on the basis of outer diameters of the preform measured at the two positions.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-115097
Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-167745